The technical terms and abbreviations used in this document correspond to the technical terminology used within the framework of “Controller Area Network Systems,” “CAN” systems for short.
The use of many electronic control units (ECUs) in motor vehicles, but in other areas as well, requires a communication system with high data throughput. The requirements of data communication have hitherto been able to be extensively satisfied with the Controller Area Network (CAN) introduced by Bosch and described in the “CAN Specification Version 2.0, September 1991.” The above-mentioned specification can be downloaded free of charge, for example, from http://www.can.bosch.com/docu/can2spec.pdf. The embodiment of new functions by the electronic unit (e.g., vehicle control systems) requires communication functions that go beyond the limits of the existing CAN systems. The data rate of current CAN networks is extensively limited by the delays in the bus drivers combined with the data lines. Concretely, the limiting factor is the need to synchronize all the CAN communication nodes located at the CAN bus to a fraction of the duration of one bit cell. Several protocol features specified by CAN Specification Version 2.0, which must be complied with by all CAN communication nodes, require simultaneity of CAN protocol-conform actions of the CAN communication nodes. This applies, in particular, to the acknowledgement of receipt common for the CAN protocol, the so-called acknowledge, and to arbitration.
FIG. 1 schematically shows the CAN message transmission process in a conventional CAN communication system. The transmission directions are indicated by arrows. A plurality of CAN communication nodes KK1, KK2 transmit their “CAN frames” simultaneously via a CAN bus, beginning with an arbitration field ARB with an “identifier segment” in order to jointly arrive at the decision on which CAN communication node KK1, KK2 is actually authorized to send a complete CAN message NAR. After the CAN communication node KK1 thus determined has transmitted data content DATA and check sum CRC of the CAN message NAR, all the CAN communication nodes KK2 active at the bus confirms the correctness of the check sum by a dominant level PEG during the acknowledgement phase of the message NAR. The term acknowledgment phase is used here synonymously for acknowledgment field (ACK field) or acknowledgment slot (ACK slot). If an error is recognized in the transmission by a receiver, this generates and sends an error frame.
The drawback of the conventional CAN communication systems is above all that the data rate that can be reached is substantially limited by the synchronization requirements resulting from the acknowledge process and the arbitration process and by the bidirectional message exchange during these phases.